# $ABC = "abc";
$def;
#var_dump (Isset ($ABC));
Var_dump (Empty ($def));
Var_dump (FALSE);
Var_dump (Empty (")");
Var_dump (Isset ($def));
Var_dump (Is_null ($def));
#var_dump (Isset ($def));
#1. PHP Data variables have declarations and definitions that are similar to variables in C/
#php中没有声明, only undefined and defined
#2. What is the definition of a PHP variable? is $def; or $ABC = "abc";?
# in PHP the definition of variables is to write variables directly to the variable assignment, that is $ABC = "abc"; This is called variable definition.
# if $def, what will it show? Let's do an experiment. Var_dump ($DEF);
#输出:
#Notice: Undefined variable:def in C:\Users\lsqbupt\Desktop\abcdefg.php on line 9
#NULL
# $def; This is called a variable not defined
#让我们再做一个实验
# if $def = null; Var_dump ($DEF);
# will output NULL, and will not have notice
#
#3. Isset () Is_null () empty () and the variables themselves as a judgment when if ($ABC) or if ($def), what is the difference between these functions?
#4. What is NULL? Is it case sensitive?
#NULL Empty Type
#null大小写不敏感, the null type has only one value, indicating that a variable has no value, and the following three case variables are considered null
#1. Assigned value is NULL
#2. Has not been assigned a value
#3. be unset ();
The function of the #is_null () function is literally visible, quite simple.
#isset () and Is_null () are a pair
#isset () function: When {undefined, such as $def} and {The variable itself is null, such as $def = NULL;} returns false and neither is notice, the other returns True
#empty () Function: When "," ", 0, ' 0 '," 0 ", NULL, False, array (), undefined variable, all return to True
The #is_null () function, when {undefined, such as $def} and {The variable itself is null, such as $def = NULL;} returns True when the other returns false
The #is_null () function pairs {Undefined, such as $def} will have notice, and the {variable itself is null such as $def = null;} But there is no notice {I found this function and Isset () is a pair of AH}
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The above introduces the variables in PHP, including the aspects of the content, I hope that the PHP tutorial interested in a friend helpful.
